The present invention relates to adsorbent beds, and in particular relates to systems for restraining particulates in the adsorbent beds that are subject to the forces of gas or fluid flow. The particulates are restrained against movement resulting, for example, in fluidization of the adsorbent bed. More particularly, in a cylindrical adsorbent bed, a bladder at an end of the adsorbent bed applies pressure to the adsorbent packing of the bed to restrain the packing, i.e. to keep the adsorbent packing from moving and becoming fluidized.
In an adsorber apparatus, the flow capacity of adsorbent beds is limited by the flow at which the bed becomes fluidized. When the bed becomes fluidized the adsorbent begins to degrade and channeling causes the efficiency of the adsorption process to drop considerably. When an adsorbent bed is properly restrained fluidization will not occur and the capacity of the adsorption process will thereby not be limited by this point. Restraining the bed therefore will permit a significant increase in flow capacity and thus a corresponding reduction in capital costs.
Improvements in gas separation process performance are currently being realized as a result of enhanced adsorption rate. Furthermore, adsorption rate considerations have resulted in a number of configurations calling for small particles throughout the adsorber as well as layers or mixtures of particles of different average particle size. For example, it is known that using particles having a larger size can reduce fluidization and minimize pressure drop. On the other hand, smaller particles are preferred for overcoming limitations to process performance such as, for example, the adsorption rate. However, fluidization is more likely to occur at decreasing flow velocity as the size of particles is decreased, thereby limiting the particle size for a process conducted at given flow rate, i.e., particles of a particular size establish a fluidization limit, a maximum flow velocity which if exceeded requires either the reduction in flow velocity or the constraint of the particles at one or both ends of the bed. Fluidization can be eliminated or reduced for all particle sizes in axial flow adsorbers if the free adsorbent surface of the bed is constrained. Other reasons for constraining adsorbents in axial flow adsorbers include elimination of scouring of the top of the bed due to high velocity purge or repressurization flows, shop-loading of adsorber vessels and prevention of xe2x80x9cbumpingxe2x80x9d or temporary lifting due to pressure disturbances from valve operations.
Apparatus used for pressure swing adsorption cycles generally employ cylindrical vessels adapted for either a radial flow or an axial flow pattern. Vessels adapted for an axial flow pattern are often preferred due to a simple construction which avoids a number of problems or design complexities inherent with vessels adapted for a radial flow pattern. However, with axial flow the limitation of fluidization of the bed must be addressed.
Adsorbent particles are inherently constrained in most vertically oriented radial flow adsorbers as the flow is lateral to the vertical axis. This makes the gravity vector normal to the flow velocity vector, and allows for relatively easy installation of an adsorbent restraint assembly.
In contrast, axial flow adsorbers have the velocity and gravity vectors aligned, which has historically made the installation of an adsorbent restraint assembly relatively difficult. Therefore for example in upflow axial flow vessels the top surface of the adsorbent bed is typically unobstructed leaving adsorbent particles free to fluidize under sufficient lifting conditions, e.g., a volume or a velocity of fluid sufficient to overcome the force of gravity acting on the particles. Fluidization of this nature is also a concern with downflow axial beds, in which regeneration and desorption flows would subject the particles to possible fluidization.
Faster cycles and smaller bed size are also desirable, particularly in the design of vacuum pressure swing adsorption (VPSA) systems. To achieve this design objective, feed velocities are increased. Present axial bed oxygen VPSA systems operate with an average superficial feed velocity of, for example, 0.15-0.3 m/sec for a bed size of 600-800 pounds of adsorbent per ton per day of oxygen. This feed rate and the corresponding bed design results in operation of the system at near fluidization levels. In fact, overlapping countercurrent equalization flow steps have been incorporated into the process at the beginning of the feed step to help restrain the adsorbent under the high initial feed flow.
As a result of operation at levels close to fluidization, some commercial beds have experienced bed fluidization due to valve failure or poorly chosen cycle tuning parameters. This fluidization disturbs the uniform dense packing of large sections of the adsorber bed, resulting in subsequent gas flow maldistribution and associated poor process performance. Thus, a simple, effective restraint system for the top surface of a particulate adsorbent bed in an adsorber having an axial flow pattern would be desirable to improve adsorption rates.
In addition to particle restraint within an adsorber vessel chamber, it is also desirable to provide for uniform flow through the adsorber bed, to eliminate unnecessary void volume and to provide full access to the interior of the vessel. Uniform flow through the adsorber bed ensures that the gas or liquid material being treated is uniformly exposed to the adsorbent particles. Eliminating unnecessary void volume reduces the loss of processed product or unprocessed feed that is trapped in the apparatus after the adsorption process is complete. Full access to the interior of the vessel permits the apparatus to be maintained, and permits the sieve material to be maintained, loaded or changed.
A number of approaches to solve or circumvent the adsorbent bed fluidization problem have been used or proposed. Flow direction, special packings, and restraints of various designs are among the prior art solutions to this problem. Related prior art includes the following patents.
In U.S. Pat. No. 5,492,684, Buchanan et al. describe a method and system for the removal of contaminants such as sulfur oxides from waste gages using a graded-bed system. The graded-bed system uses beds with solid sorbents of two or more particle sizes in separate sections of the bed. In one embodiment the solid sorbents are arranged so the larger sorbent particles are disposed in the entrance region of the graded-bed system. In operation, a waste gas stream is passed over and through the solid sorbents so that contaminants, such as sulfur oxides and/or nitrogen oxides are adsorbed. The sorbent bed is then contacted with a reducing gas to desorb the sulfur oxides. The use of expandable means applied to the bed to insure the stability of the bed is not disclosed.
U.S. Pat. No. 4,337,153 to Prior discloses an improved resin tank for a water softening apparatus including an expandable chamber that enlarges during fluid flow through the tank to displace any free space in the tank, thereby maintaining the compactness of the water softening material. The expandable chamber is formed by an elastomeric sleeve that is secured to and surrounds a portion of a downward extending fluid conduit and overlies at least one aperture formed in the conduit wall through which fluid communication is established. The pressure drop normally occurring during fluid flow through the tank generates a pressure differential on the sleeve wall that causes it to enlarge if there is free space in the tube. In an alternate embodiment, the fluid communication between the conduit and the chamber is provided by a pitot tube that is disposed in the conduit fluid flow path and is operative to communicate the velocity pressure of the fluid flowing down the conduit to the chamber.
A bed of particulate ion-exchange material, in U.S. Pat. No. 4,294,699 by Hermann, is confined in the cavity of a container bound by a wall portion of the container which is movable inward of the cavity. Supply and discharge conduits communicate with respective portions of the cavity for supplying the liquid to be purified, and for discharging from the cavity the liquid purified by contact with the particulate material. A biasing device engages the container outside the cavity and biases the movable wall portion inward of the cavity, whereby the particulate material is kept under compressive stress, and channeling due to shrinkage of the bed is avoided.
A process for continuous countercurrent contact with a magnetically stabilized fluidized bed is described by Coulaloglou and Siegell in U.S. Pat. No. 4,247,987. The patent relates to the operation of a magnetically stabilized bed with continuous solid addition and removal. The bed particles, which include a magnetizable component, are stabilized against gas by-passing and solid back-mixing (except possibly for the time flow or movement of the solids near the entrance or exit ports or near fluid injection zones) during countercurrent contacting by the use of an applied magnetic field. This is particularly suited for carrying out separation processes. The use of applied magnetic fields in such processes enables one to use small size fluidizable adsorbent particles without encountering high pressure drops. The small adsorbent particles having a magnetic component give faster transfer of the sorbed species from the contacting fluid than with larger adsorbent particles which allows for a closer approach to equilibrium.
An extensible sleeve surrounding a central tubular member to prevent fluidization in an adsorbent bed is disclosed in U.S. Pat. No. 4,997,465 to Stanford. A fluid amplifier amplifies the fluid pressure from the system gases to expand the extensible sleeve. Particles of the zeolite adsorbent are inhibited from becoming fluidized and moving with fluid flows by the clamping pressure between the extensible sleeve and the outside wall of the containing vessel.
U.S. Pat. No. 5,176,721, to Hay et al. discloses an adsorber apparatus with a vertical cylinder in which particulate adsorbers are arranged in vertically oriented layers between two vertically oriented perforated parallel panels extending within a vessel and spaced from one another to define an adsorbent mass chamber. The adsorbent is arranged in two vertical layers, a first part of fine particles and a second layer of larger particles. The gas to be treated is circulated horizontally from one perforated panel to the other perforated panel through the adsorbent layers. A flexible membrane at one end of the wall defines a separate end chamber sealed from the adsorber chamber. A small pipe supplies pressure or vacuum to the end chamber. The particles in the adsorber chamber are restrained between the vertical panels in a relatively fixed position under the effect of compression by the membrane or diaphragm when pressure is maintained in the end chamber at least equal to the highest pressure in the adsorption chamber in an adsorption cycle.
It is among the objects of this invention to provide uniform restraint of adsorbent particles in an axial flow adsorbent bed thereby preventing movement and subsequent degradation of the particles while maintaining high process efficiency.
It is another object of the invention to reduce the void volume of an adsorber vessel thereby improving the adsorption cycle efficiency by reducing the volume of product quality gas remaining in the adsorber vessel after the adsorption step is complete.
In accordance with the present invention, an adsorber apparatus is provided, comprising a vessel having a peripheral wall and first and second end walls defining a chamber, the peripheral wall defining a cross-section of the chamber and the chamber having an inlet end adjacent the first end wall and an outlet end adjacent the second end wall, and defining a longitudinal axis extending from the inlet end to the outlet end of the wall. The apparatus has an inlet port in fluid communication with its inlet end for supplying a fluid mixture to the vessel, and an outlet port in fluid communication with the outlet end for evacuating a separated gas out of the vessel.
A quantity of particulate adsorbent (either bead, extrudate or granular) is charged to and fills a substantial portion of the chamber between the inlet end and the outlet end of the adsorber vessel, a surface of the quantity of particulate adsorbent corresponding dimensionally to the chamber cross-section and facing its outlet end, the surface being formed of a plurality of individual particles each having a minimum dimension. A porous barrier substantially covers the surface, the barrier being movable with the surface along the longitudinal axis of the chamber, and the barrier being adapted to restrain the individual particles from entering the outlet end and adapted to permit the separated gas product to enter the outlet end of the chamber.
A plurality of balls (or similar objects) of graded sizes is further provided in the outlet end positioned between the barrier and the second end wall. Finally, a bladder is positioned between the plurality of balls and the second end wall, the bladder being adapted to be pressurized to bias the barrier against the surface such that the quantity of particulate adsorbent is held with a compressive force sufficient to prevent fluidization of the adsorbent.
Employing the adsorber of the invention, the void volume of the adsorber vessel is reduced thereby improving the adsorption cycle efficiency. The installation of the restraint bladder above the adsorbent bed in a manner that eliminates unnecessary void under the top head, and the use of the graded balls directly above the adsorbent bed minimizes the top void in the axial bed vessel. The unwanted void reduces process efficiency by storing product quality gas in the adsorber after the adsorption step is complete which escapes as inefficient reflux gas during the evacuation step.
The invention provides for uniform gas flow through the adsorbent bed. The design of the hold down bladder under the top head is such that the necessary hold down force is exerted on the adsorbent while the shape is such that uniform gas flow is maintained through the head space and, subsequently, the adsorbent bed. The grading of the balls under the bladder is such that pressure gradients are designed to produce uniform gas flow through the adsorbent bed.
This design of the bladder allows for full access to the vessel to load or change sieve material. Proper loading of the adsorbent material is necessary in order to maintain uniform pressure drop and therefore uniform flow through the adsorbent bed. In order to achieve this loading characteristic, the top head space must be accessible during the loading period utilizing a particle loader such as described in U.S. Pat. No. 5,324,159 to Nowobilski et al. This bladder system and graded ball system provides for this accessibility.